Heat pipe with sintered powder wick

ABSTRACT

A heat pipe ( 10 ) includes a casing ( 12 ) and a sintered powder wick ( 14 ) arranged at an inner surface of the casing. The sintered powder wick is in the form of a multi-layer structure in a radial direction of the casing and at least one layer is divided into multiple sections in a longitudinal direction of the casing, and the multiple sections have powder sizes different from each other. The sections with large-sized powders are capable of reducing the flow resistance to the condensed liquid to flow back while the sections with small-sized powders are capable of providing a satisfactory capillary force for moving the condensed liquid.

FIELD OF THE INVENTION

The present invention relates generally to apparatus for transfer or dissipation of heat from heat-generating components such as electronic components, and more particularly to a heat pipe having a sintered powder wick.

DESCRIPTION OF RELATED ART

Heat pipes have excellent heat transfer performance due to their low thermal resistance, and therefore are an effective means for transfer or dissipation of heat from heat sources. Currently, heat pipes are widely used for removing heat from heat-generating components such as central processing units (CPUs) of computers. A heat pipe is usually a vacuum casing containing therein a working fluid, which is employed to carry, under phase transitions between liquid state and vapor state, thermal energy from one section of the heat pipe (typically referring to as “evaporating section”) to another section thereof (typically referring to as “condensing section”). Preferably, a wick structure is provided inside the heat pipe, lining an inner wall of the casing, for drawing the working fluid back to the evaporating section after it is condensed at the condensing section. Specifically, as the evaporating section of the heat pipe is maintained in thermal contact with a heat-generating component, the working fluid contained at the evaporating section absorbs heat generated by the heat-generating component and then turns into vapor. Due to the difference of vapor pressure between the two sections of the heat pipe, the generated vapor moves towards and carries the heat simultaneously to the condensing section where the vapor is condensed into liquid after releasing the heat into ambient environment by, for example, fins thermally contacting the condensing section. Due to the difference of capillary pressure developed by the wick structure between the two sections, the condensed liquid is then drawn back by the wick structure to the evaporating section where it is again available for evaporation.

The wick structure currently available for heat pipes includes fine grooves integrally formed at the inner wall of the casing, screen mesh or bundles of fiber inserted into the casing and held against the inner wall thereof, or sintered powder combined to the inner wall of the casing by sintering process. Among these wicks, the sintered powder wick is preferred to the other wicks with respect to heat transfer ability and ability against gravity of the earth.

In a heat pipe, the primary function of a wick is to draw the condensed liquid back to the evaporating section of the heat pipe under the capillary pressure developed by the wick. Thus, the capillary pressure has become an important parameter to evaluate the performance of the wick. Since it is well recognized that the capillary pressure of a wick increases due to a decrease in pore size of the wick, the sintered powder wick generally has a capillary pressure larger than that of the other wicks due to its very dense structure of small particles. In order to obtain a relatively large capillary pressure for a sintered powder wick, small-sized powders are often used so as to reduce the pore size formed between the powders. However, it is not always the best way to choose a sintered powder wick based on the size of powder, because the flow resistance to the condensed liquid also increases due to a decrease in pore size of the wick. The increased flow resistance significantly reduces the speed of the condensed liquid in returning back to the evaporating section and ultimately limits the heat transfer performance of the heat pipe. As a result, a heat pipe with a wick that has a too large or a too small pore size often suffers dry-out problem at the evaporating section as the condensed liquid cannot be timely sent back to the evaporating section of the heat pipe.

Therefore, it is desirable to provide a heat pipe with a sintered powder wick that can provide a satisfactory capillary force and a reduced flow resistance to the condensed liquid so as to effectively and timely bring the condensed liquid back to the evaporating section of the heat pipe and thereby avoid the undesirable dry-out problem at the evaporating section.

SUMMARY OF INVENTION

A heat pipe in accordance with a preferred embodiment of the present invention includes a casing and a sintered powder wick arranged at an inner surface of the casing. The sintered powder wick is in the form of a multi-layer structure in a radial direction of the casing and at least one layer is divided into multiple sections in a longitudinal direction of the casing, and the multiple sections have powder sizes different from each other.

The present invention in another aspect, relates to a method for manufacturing a sintered heat pipe. The preferred method includes steps of: (1) providing a hollow casing; (2) inserting a mandrel into the casing, and filling powders into said casing to form a layer of powder under the control of the mandrel; (3) pre-sintering the layer of powder at a first temperature and drawing out the mandrel; (4) repeating the steps of (2) and (3) until at least two layers of powder are formed inside the casing, wherein in forming one of the at least two layers of powder, at least two groups of powder with different powder sizes are used; and (5) sintering said at least two layers of powder at a second temperature higher than the first temperature, whereby a sintered powder wick with a multi-layer structure is formed inside the heat pipe.

Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of a heat pipe in accordance with a first embodiment of the present invention;

FIG. 2 is a longitudinal cross-sectional view of a heat pipe in accordance with a second embodiment of the present invention;

FIG. 3 is a longitudinal cross-sectional view of a heat pipe in accordance with a third embodiment of the present invention; and

FIGS. 4A-4C are longitudinal cross-sectional views showing the steps of a preferred method in manufacturing the heat pipe of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates a heat pipe 10 in accordance with a first embodiment of the present invention. The heat pipe 10 includes a casing 12 and a capillary wick 14 arranged at an inner surface of the casing 12. The casing 12 includes an evaporating section 121 and a condensing section 123 at respective opposite ends thereof, and a central section 122 located between the evaporating section 121 and the condensing section 123. The casing 12 is typically made of high thermally conductive materials such as copper or copper alloys. The wick 14 is saturated with a working fluid (not shown), which acts as a heat carrier for carrying thermal energy from the evaporating section 121 toward the condensing section 123 when undergoing a phase transition from liquid state to vaporous state. In more detail, heat that needs to be dissipated is transferred firstly to the evaporating section 121 of the heat pipe 10 to cause the working fluid saturated in the wick 14 to evaporate. Then, the heat is carried by the working fluid in the form of vapor to the condensing section 123 where the heat is released to ambient environment, thus condensing the vapor into liquid. The condensed liquid is then brought back, via the wick 14, to the evaporating section 121 where it is again available for evaporation.

The capillary wick 14 is a sintered powder wick which is formed by sintering small-sized powders, such as metal powders including copper and aluminum, or ceramic powders under high temperature. Along a radial direction of the casing 12, the wick 14 has a three-layer structure, which includes in sequence an outer layer 141, an intermediate layer 142 and an inner layer 143. These layers 141, 142, 143 are stacked together along the radial direction of the casing 12 with the outer layer 141 being connected to the inner surface of the casing 12. Furthermore, each layer of the wick 14 is divided into three sections along a longitudinal direction of the casing 12. For example, as for the inner layer 143, it includes sequentially a first section 1431, a second section 1432 and a third section 1433 along the direction from the evaporating section 121 to the condensing section 123. The three sections of each layer of the wick 14 have powder sizes different from one another, and these sections are consistent in length with the evaporating, central and condensing sections 121, 122, 123 of the casing 12, respectively. As for the inner layer 143, the first section 1431 has the largest powder size, whereas the third section 1433 has the smallest powder size. Moreover, every three sections of the three layers 141, 142, 143 of the wick 14 that correspond to a single section of the casing 12 in the radial direction thereof also have powder sizes different from one another. Thus, in this embodiment, every three consecutive sections of the wick 14, viewing in either the radial direction or longitudinal direction of the casing 12, have powder sizes different from each other.

Since the powder size of the powders of the wick 14 also determines the pore size formed between the powders, the pore sizes defined by every three consecutive sections of the wick 14 also differ from one another. According to the general rule that the capillary pressure of a wick and its flow resistance to the condensed liquid increase due to a decrease in pore size of the wick, the sections that have large powder sizes among every three consecutive sections, for example, the sections 1431, 1432 of the inner layer 143, are capable of providing a reduced flow resistance to the condensed liquid due to having relatively large pore sizes, thereby reducing the resistance the condensed liquid encounters when flowing through these sections. However, the other section that has small powder size among the three consecutive sections, for example, the section 1433 of the inner layer 143, is still capable of maintaining a relatively high capillary force for the wick 14. Thus, the multi-layer and multi-section structure of the wick 14 is thus capable of simultaneously providing a satisfactory capillary force and a reduced flow resistance for the condensed liquid. As the flow resistance to the condensed liquid is reduced, the condensed liquid is therefore capable of being timely brought back to the evaporating section 121 of the heat pipe 10, effectively avoiding the dry-out happening at that section.

As illustrated in the first embodiment, the three layers 141, 142, 143 of the wick 14 almost have identical thicknesses and each section of every layer almost exactly covers the corresponding section of the casing 12 in length. However, some other configurations may also be suitable for the wick 14 of the heat pipe 10. For example, according to a second embodiment of the present invention as illustrated in FIG. 2, a heat pipe 20 is shown that has a multi-layer sintered powder wick 24 with each layer having a different thickness, the inner layer 243 being the thinnest, the intermediate layer 242 being the thickest and the outer layer 241 having an intermediate thickness therebetween. Similarly, according to a third embodiment of the present invention, FIG. 3 illustrates a heat pipe 30 in which the sections of two adjacent layers of the wick are arranged in a staggered manner. In other words, the sections of neighboring layers of the wick are constructed to have different lengths. However, it should also be recognized that it is not necessary that all of the layers 141, 142, 143 of the wick 14 be in a multi-section configuration. Some of them may also have a uniform structure, i.e., having a uniform powder size and therefore a uniform pore size distribution through their entire lengths.

The heat pipe 10 as disclosed in the first embodiment can be made by using the method as illustrated in FIGS. 4A-4C. In order to form the multi-layer and multi-section wick 14, multiple groups of powder with different powder sizes are prepared in advance. First of all, a first mandrel 40 a is inserted into the casing 12 with a space (not labeled) formed between the casing 12 and the first mandrel 40 a. Then, three groups of powder with different powder sizes from each other are sequentially filled into the space with the later filled group of powder being stacked on the earlier filled group of powder along the longitudinal direction of the casing 12, thereby defining a first layer of powder that is to be constructed as the outer layer 141 of the wick 14, as shown in FIG. 4A. The first mandrel 40 a is used to control the thickness of the first layer of powder. Thereafter, second and third mandrels 40 b, 40 c with smaller diameters than that of the first mandrel 40 a are respectively and successively used, as the powders with different sizes are continued to be filled into the casing 12, to control and form second and third layers of powder, which are to be respectively constructed as the intermediate and inner layers 142, 143 of the wick 14, as shown in FIGS. 4B-4C. In order to keep the powders in place and prevent, after the corresponding mandrel 40 a, 40 b or 40 c is drawn out, the powders from dropping into the hollow space that is originally occupied by the corresponding mandrel 40 a, 40 b or 40 c, each layer of powder is pre-sintered by heating the layer of powder with the corresponding mandrel 40 a, 40 b or 40 c and the casing 12 at a suitable temperature. The suitable temperature is, for example, about 630 degrees Celsius when the powders are copper powders. After the pre-sintering, the corresponding mandrel 40 a, 40 b or 40 c used to control the thickness thereof is drawn out of the casing 12. Finally, after the powders necessary to construct the wick 14 are all filled into the casing 12 and pre-sintered, the casing 12 with the powders is subject to heat with a high temperature, fox example, about 950 degrees Celsius when the powders are copper powders, to thereby sinter the powders together whereby the heat pipe 10 with the multi-layer and multi-section wick 14 arranged along the inner surface of the casing 12 is obtained. In this method, in order to prevent the later filled small-sized powders from falling into spaces defined between particles of the former filled large-sized powders, partitioning means such as a layer of polymeric bonding agent can be applied over the interface between every two adjacent groups of powder. The bonding agent can be decomposed by subsequently applying heat thereto.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

1. A heat pipe comprising: a casing; and a sintered powder wick arranged at an inner surface of the casing; wherein the sintered powder wick is in the form of a multi-layer structure in a radial direction of the casing and at least one layer is divided into multiple sections in a longitudinal direction of the casing, and the multiple sections have powder sizes different from each other.
 2. The heat pipe of claim 1, wherein at least two layers of the sintered powder wick are divided into multiple sections and the sections of the two layers are constructed to have identical lengths.
 3. The heat pipe of claim 1, wherein at least two adjacent layers of the sintered powder wick are divided into multiple sections and at least two adjacent sections of the two layers in said radial direction have powder sizes different from each other.
 4. The heat pipe of claim 1, wherein at least two layers of the sintered powder wick are divided into multiple sections and the sections of the two layers are constructed to have different lengths.
 5. The heat pipe of claim 1, wherein the multiple layers of the sintered powder wick have different thicknesses.
 6. The heat pipe of claim 1, wherein each layer of the sintered powder wick is divided into multiple sections.
 7. The heat pipe of claim 6, wherein the sintered powder wick has a three-layer structure and each layer is divided into three sections.
 8. The heat pipe of claim 1, wherein the sintered powder wick is one of a sintered metal powder wick and a sintered ceramic powder wick.
 9. A method for manufacturing a heat pipe comprising steps of: (1) providing a hollow casing; (2) inserting a mandrel into the casing, and filling powders into said casing to form a layer of powder under the control of the mandrel; (3) pre-sintering the layer of powder at a first temperature and drawing out the mandrel; (4) repeating the steps of (2) and (3) until at least two layers of powder are formed inside the casing, wherein in forming one of the at least two layers of powder, at least two groups of powder with different powder sizes are used; and (5) sintering said at least two layers of powder at a second temperature higher than the first temperature, whereby a sintered powder wick with a multi-layer structure is formed inside the heat pipe.
 10. The method of claim 9, wherein the powders are one of metal powders and ceramic powders.
 11. The method of claim 9, wherein the powders are copper powders and the first temperature is about 630 degrees Celsius.
 12. The method of claim 9, wherein the powders are copper powders and the second temperature is about 950 degrees Celsius.
 13. The method of claim 9, wherein when forming each of the at least two layers of powder, said at least two groups of powder with different powder sizes are used.
 14. A heat pipe for transmitting heat from one section of the heat pipe to another section of the heat pipe comprising: a metal hollow casing having an inner surface; and a sintered powder wick in the casing and adjacent to the inner surface, the wick comprising a plurality of layers along a radial direction of the casing and a plurality of sections along a longitudinal direction of the casing, wherein two neighboring sections of one of the layers have different pore sizes.
 15. The heat pipe of claim 14, wherein two neighboring sections of two neighboring layers have different pore sizes.
 16. The heat pipe of claim 14, wherein the heat pipe has an evaporating section and a condensing section, and one of the sections of the sintered powder wick is located and has a length corresponding to the evaporating section and another of the sections of the sintered powder wick is located and has a length corresponding to the condensing section.
 17. The heat pipe of claim 14, wherein the layers have different thickness.
 18. The heat pipe of claim 14, wherein the sections have different lengths.
 19. The heat pipe of claim 17, wherein the sections have different lengths. 